Synthesis gas, or "syngas," is a mixture of gases prepared as feedstock for a chemical reaction; for example, carbon monoxide and hydrogen to make hydrocarbons or organic chemicals, or hydrogen and nitrogen to make ammonia. Syngas may be produced for use with a Fischer-Tropsch process, which is described further below and which is used as an example throughout.
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is known and is generally referred to as the Fischer-Tropsch reaction. Numerous catalysts have been used in carrying out the reaction, and at relatively low to medium pressure (near atmospheric to 600 psig) and temperatures in the range of from about 300.degree. F. to 600.degree. F., both saturated and unsaturated hydrocarbons can be produced.
The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity. The Fischer-Tropsch reaction can be characterized by the following general reaction: EQU 2H.sub.2 +CO.fwdarw..sub.Catalyst -CH.sub.2.sup.- +H.sub.2 O
Two basic methods have been employed for producing the synthesis gas utilized as feedstock in the Fischer-Tropsch reaction. The two methods are steam reforming, wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation, wherein one or more light hydrocarbons are combusted or reacted sub-stoichiometrically to produce synthesis gas.
The basic steam reforming reaction of methane is represented by the following formula: EQU CH.sub.4 +H.sub.2 O.sub.Catalyst .fwdarw.CO+3H.sub.2
The steam reforming reaction is endothermic and a catalyst containing nickel is often utilized. The hydrogen to carbon monoxide ratio of the synthesis gas produced by steam reforming of methane is approximately 3:1.
Partial oxidation is the non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The basic reaction is represented as follows: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2
The partial oxidation reaction is typically carried out using high purity oxygen. High purity oxygen can be quite expensive. The hydrogen to carbon monoxide ratio of synthesis gas produced by the partial oxidation of methane is approximately 2:1.
In some situations these approaches may be combined. A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air is used as a source of oxygen for the partial oxidation reaction has also been used for producing synthesis gas heretofore. For example, U.S. Pat. Nos. 2,552,308 and 2,686,195 disclose low pressure hydrocarbon synthesis processes wherein autothermal reforming with air is utilized to produce synthesis gas for the Fischer-Tropsch reaction. Autothermal reforming is a combination of partial oxidation and steam reforming where the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby low cost is typically involved.
The autothermal process results in a lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, as stated above, the steam reforming reaction with methane results in a ratio of about 3:1 while the partial oxidation of methane results in a ratio of about 2:1. The optimum ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure over a cobalt catalyst is 2:1. When the feed to the autothermal reforming process is a mixture of light hydrocarbons such as a natural gas stream, some form of additional control is desired to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio of about 2:1.
In producing a product from the synthesis unit, a residue gas is frequently produced. For some processes, the use of this gas to create energy has been suggested. Systems that have utilized the residue gas have required numerous additional components and steps to do so.
In producing a synthesis gas for the Fischer-Tropsch process or any other process, it is desirable to produce the synthesis gas as efficiently as possible. The ability to develop a process with low capital expense may be an imperative to development of large-scale systems.